1. Field of the Invention
The present invention relates to a crack-type fatigue detecting sensor, a method for fabricating the crack-type fatigue detecting sensor, and a method for estimating fatigue damage using the crack-type fatigue detecting sensor, which are suitably practiced for measuring damage due to fatigue of various members of a structural component such as a bridge, a machine, a vehicle, an air plane, and the like.
2. Description of the Related Art
A typical prior art is disclosed in Japanese Laid-Open Patent Publication No. Sho. 62-265558. This prior art discloses a crack-type fatigue detecting sensor in which a fracture piece having a slit causing fatigue damage is fixed to a surface of a plate-shaped substrate, a method for fabricating a crack-type fatigue detecting sensor in which opposite end portions (in a longitudinal direction) of a fracture piece having a slit are bonded to a surface of a substrate by bond for fixation, and a method for estimating damage using a crack-type fatigue detecting sensor in which two crack-type fatigue detecting sensors having different crack propagation characteristics are fixed to a member to be tested, lengths of respective cracks propagating during the same period are measured, and damages occurring in the period, due to fatigue of the member to be tested, which are associated with these lengths of crack propagation, are estimated.
Another prior art is disclosed in Japanese laid-Open Patent Publication No. Hei. 9-304240. In this prior art, a thin-plate shaped fracture piece made of a material identical to a material of a structural component for which fatigue damage is predicted and having a slit in a central portion in a longitudinal direction thereof is sandwiched between two synthetic resin thin plates and these are bonded in a region except the central portion of the fracture piece that includes the slit. This test piece is made of the material identical to that of the member to be tested. The crack-type fatigue detecting sensor is fabricated in such a manner that a member having a circular hole in the central portion in the longitudinal direction thereof and the slit extending from the circular hole toward both ends in a width direction thereof is sliced into pieces having a thickness equal to a thickness of the fracture piece, and so formed fracture piece is sandwiched between the two synthetic resin thin plates and the opposite end portions of the fracture piece in the longitudinal direction thereof are bonded to the thin plates. This prior art also discloses a method for estimating damage using the crack-type fatigue detecting sensor, in which the crack-type fatigue detecting sensor is fixed at a position apart from a portion at which stress concentrates, which is so-called a hot spot such as a weld toe, and life of the member to be tested is estimated based on an Sxe2x88x92N (=stressxe2x88x92number of repeated load cycles) diagram for the member to be tested created in advance.
Still another prior art is disclosed in Japanese Laid-Open Patent Publication No. Hei. 10-185854. In this prior art, a crack-type fatigue detecting sensor having a plurality of strain gauges spaced apart from one another and placed either in parallel or in series in a direction perpendicular to a direction of a crack of fatigue damage occurring on the member to be tested is attached to the member to be tested, a value of a length of crack propagation occurring on the fracture piece is electrically measured, and damage due to fatigue of the member to be tested is estimated based on this measured value.
A further prior art is disclosed in Japanese Patent No. 2952594. In this prior art, a fracture piece having a slit is provided with strain gauges or crack gauges obtained by placing a plurality of electric resistance wires in parallel with one another in a direction orthogonal to a direction in which a crack initiating from a tip end portion of the slit propagates, for facilitating measurement of a length of crack propagation occurring on the fracture piece, and based on the measured length, damage due to fatigue is estimated.
In the prior art disclosed in the Japanese Laid-Open Patent Publication No. Sho. 62-265558, since the fracture piece, on which no tensile stress remains, is fixed to the substrate, strain occurring on the member to be tested is transmitted to the fracture piece through the substrate, causing the crack to occur at the tip end portion of the slit of the fracture piece, and from the length of the crack, damage due to fatigue of the member to be tested is measured. Therefore, the fracture piece requires tensile stress large enough to cause the crack at the tip end portion of the slit, and strain of the member to be tested that is too small to cause any crack cannot be detected. As a result, sensitivity is low.
In the prior art disclosed in the Japanese laid-Open Paten Publication No. Sho. 9-304240, the fracture piece is sandwiched between two synthetic resin thin plates such that the opposite end portions of the fracture piece in the longitudinal direction are joined to these plates, and one of the thin plates is fixed to the member to be tested. Therefore, the strain occurring on the member to be tested is transmitted to the fracture piece through the thin plate. The strain occurring on the member to be tested is not reliably transmitted to the fracture piece because a part of the strain is absorbed in the thin plate. As a result, sensitivity is low. The crack-type fatigue detecting sensor of this prior art has a large outer shape (70 mm long, 20 mm wide, and 1.5 mm thick). For this reason, this sensor cannot be attached to the member to be tested in proximity to a peripheral end of, for example, a welded bead of this member. Therefore, a position at which damage is measured is limited. Under the circumstance, it is highly probable that the aim cannot be achieved.
In the prior art disclosed in the Japanese laid-Open Patent Publication No. Hei. 10-185854, without an element corresponding to a substrate of the present invention, the fracture piece is directly attached to the member to be tested and the length of crack propagation occurring on the fracture piece is electrically measured by an electric means such as the strain gauge or the electric resistance wire and monitored. The fracture piece has a fixed thickness and a large outer shape (170 mm long, 50 mm wide, and 0.5 mm thick) and the position at which the fatigue damage of the member to be tested is measured is extremely limited.
In the prior art disclosed in the Japanese Patent No. 2952594, although the following two respects are devised to improve the strain sensitivity of the crack initiating from the slit of the sensor attached to the member to be tested, problems associated with its cost and practical use arise. i) Because the crack is difficult to occur in a condition in which only grooves are formed, load is repeatedly subjected to cause the fatigue crack to occur and its tip end portion is made sharp. Still, the crack is difficult to occur under the influence of the compressive stress remaining at the tip end portion of the crack. So, heat treatment (residual stress relief annealing) is carried out to reduce the residual stress. This results in enormous labor and high cost in fabrication. ii) When directly attaching the sensor to the member to be tested, tensile residual stress is given to the sensor. This is sometimes impossible in practice and its management is extremely difficult.
An object of the present invention is to provide a crack-type fatigue detecting sensor which has a compact configuration and is capable of measurement at a position selected with improved degree of freedom and with high sensitivity, high precision, and high reliability, a method for fabricating the crack-type fatigue detecting sensor, and a method for estimating damage using the crack-type fatigue detecting sensor.
To achieve the above-described problem, there is provided a crack-type fatigue detecting sensor comprising: a foil substrate having a first surface and a second surface fixed to a member to be tested; and a foil fracture piece having a slit formed in a central portion between opposite end portions in a longitudinal direction thereof such that the slit extends from one side portion toward the other side portion in a width direction thereof perpendicular to the longitudinal direction, wherein the opposite end portions of the fracture piece are respectively fixed to the first surface of the substrate and a portion including the central portion having the slit has a thickness smaller than a thickness of the opposite end portions over a whole width.
With such a structure, the opposite end portions of the foil fracture piece having the slit formed in the intermediate portion between the opposite end portions are fixed on the first surface of the foil substrate. The slit extends in the central portion of the fracture piece from one side portion toward the other side portion in the width direction perpendicular to the longitudinal direction. The second surface of the substrate to which the fracture piece is fixed is fixed on the member to be tested. When strain of the member to be tested is transmitted to the fracture piece through the substrate, the crack propagates from the tip end of the slit. Based on the length of the crack, the fatigue damage of the member to be tested can be measured.
The fracture piece is structured such that the intermediate portion including the central portion having the slit has a thickness smaller than a thickness of the opposite end portions over the entire width. Therefore, the strain transmitted from the member to be tested to the opposite end portions through the substrate, causes large stress to be generated in the vicinity of the slit, and this large stress concentrates on the tip end of the slit. Since the large stress concentrating on the tip end of the slit can be thus generated by the strain of the member to be tested, the large stress is generated at the tip end of the slit if the strain occurring on the member to be tested is small. Thereby, fatigue damage caused by small strain of the member to be tested can be measured based on the length of the crack occurring on the fracture piece with high sensitivity, high precision, and high reliability.
It is preferable that in the detecting sensor, the facture piece with tensile stress remaining thereon is fixed to the first surface of the substrate at the opposite end portions.
With such a structure, since the tensile stress remains on the fracture piece, the small strain of the member to be tested can cause the crack to propagate from the tip end of the slit. Therefore, sensitivity can be further improved. Also, precision can be improved by setting the tensile residual stress of the fracture piece to have a predetermined value or larger.
It is preferable that in the detecting sensor, the fracture piece is directly joined to the first surface of the substrate at the opposite end portions or indirectly joined to the first surface at the opposite end portions via a bonding layer.
With such a structure, since the opposite end portions of the fracture piece is directly joined to the first surface of the substrate or indirectly joined to the first surface via the bonding layer, the strain can be reliably transmitted from the substrate to the opposite end portions of the fracture piece and the tensile stress can be concentratively generated in the central portion of the fracture piece without being dispersed, which facilitates occurrence of the crack.
It is preferable that in the detecting sensor, the fracture piece is provided with a means for electrically measuring a length of crack propagation in a region from a tip end portion of the slit to the other side portion in the width direction, on which the crack develops.
With such a structure, since the measuring means is capable of electrically measuring the length of the crack propagation of the fracture piece, data acquisition and management associated with the crack propagation can be easily carried out. The fatigue detecting sensor is placed on the member to be tested to measure the damage continuously or intermittently on a regular basis over a long time period. Therefore, the measurement operation of the fatigue damage of the member to be tested can be easily performed.
It is preferable that in the detecting sensor, the slit formed in the fracture piece has the tip end portion that is pointed in a direction from the one side portion toward the other side portion in the width direction.
With such a structure, since the tip end of the slit is pointed in the direction from the one side portion to the other side portion in the width direction of the fracture piece, the stress can concentrate on a narrow region at the tip end portion of the slit. Thereby, occurrence of the crack can be facilitated, and sensitivity is further improved.
It is preferable that in the detecting sensor, the fracture piece has one surface and the other surface and a step face is formed between the one surface of the intermediate portion and the one surface of each of the opposite end portions such that step faces are vertically provided with the slit situated between the step faces, and a portion at which each of the step faces and the surface of the intermediate portion intersect continues in a direction toward the other surface of the fracture piece via a convex curved face.
With such a structure, the step face is formed between the surface of the intermediate portion and the surface of the opposite end portions and the convex curved face is formed at the portion at which each of the step faces and the surface of the intermediate portion intersect in the direction toward the other surface of the fracture piece. With the second surface of the substrate fixed to the member to be tested, the strain occurring on the member to be tested is transmitted to the opposite end portions of the fracture piece via the substrate, and then transmitted to the vicinity of the slit in the intermediate portion, causing the crack to occur from the tip end of the slit. Since the intersecting portion has the curved face, the stress transmitted from the opposite end portions to the intermediate portion is relieved on the intersecting portion, i.e., least concentrates thereon. Also, occurrence of the crack on the intersecting portion before occurrence of the crack on the slit is avoided, and reduction of concentration of the stress at the tip end of the slit, caused by dispersion of the stress at the intersecting portion, is minimized to make the stress concentrate on the tip end of the slit, for facilitating occurrence of the crack. As a result, sensitivity can be further improved.
It is preferable that in the detecting sensor, a ratio L3/L4 of a length L3 of the intermediate portion in the longitudinal direction to a length L4 of an unjoined region between the opposite end portions is used to adjust sensitivity in such a manner that the sensitivity is made higher as the ratio L3/L4 is decreased.
With such a structure, by suitably adjusting the length L3 of the intermediate portion with respect to the length L4 of the unjoined region to increase or decrease the ratio L3/L4, the sensitivity can be arbitrarily set. For example, when the ratio L3/L4 is decreased, the sensitivity is made higher. Therefore, the fatigue detecting sensor having high precision and desired sensitivity is realized. The fatigue detecting sensor having the desired sensitivity can be easily fabricated because the length L3 of the intermediate portion can be adjusted without special treatment.
There is also provided a method for fabricating a crack-type fatigue detecting sensor comprising the steps of: forming a foil film having a slit in a central portion between opposite end portions in a longitudinal direction of the foil film, by electroforming plating, the slit extending from one side portion toward the other side portion in a width direction of the foil film; after covering a region of a first surface of the foil film except an intermediate portion including the central portion having the slit and a second surface of the foil film with a resist film, with the intermediate portion exposed, forming a fracture piece including the intermediate portion having a predetermined reduced thickness by etching; and joining the opposite end portions of the fracture piece to the substrate.
With such a procedure, the foil film having the slit in the central portion between the opposite end portions can be formed by electroforming, the metal foil having a uniform thickness can be easily formed. The foil film is etched with the region except the surface of the intermediate portion covered with the resist film, thereby forming the fracture piece including the intermediate portion having the predetermined reduced thickness. Since the etching is employed to reduce the thickness of the intermediate portion, the intermediate portion is isotropically etched to have a uniformly reduced thickness. Thereby, the tensile stress is generated synmetrically with respect to the slit. Consequently, the fatigue detecting sensor with high reliability, on which the crack reliably propagates, is obtained.
There is further provided a method for fabricating a crack-type fatigue detecting sensor comprising the steps of: forming a foil film having a slit in a central portion between opposite end portions in a longitudinal direction of the foil film, the slit extending from one side portion toward the other side portion in a width direction of foil film; after covering a region of a first surface of the foil film except an intermediate portion including the central portion having the slit and a second surface of the foil film with a resist film, with the intermediate portion exposed, forming a fracture piece including the intermediate portion having a predetermined reduced thickness by etching; and joining the opposite end portions of the fracture piece to the substrate made of a material having a linear expansion coefficient lower than a linear expansion coefficient of a material of the fracture piece at a predetermined elevated temperature higher than a normal temperature.
With such a procedure, the fracture piece having the slit in the central portion and including the intermediate portion having the reduced thickness is joined to the substrate made of the metal material having the linear expansion coefficient lower than that of the material of the fracture piece at the opposite end portions in the longitudinal direction, at the predetermined elevated temperature higher than the normal temperature. When the fracture piece and the substrate are cooled to the normal temperature, the tensile stress can remain on the fracture piece. Because the tensile stress remains on the fracture piece, the crack can occur from the tip end of the slit and the fatigue damage can be measured with high sensitivity, if the strain occurring on the member to be tested is small. In addition, since the metal foil is formed by electroforming and the fracture piece is etched to obtain the intermediate portion having the reduced thickness, the sensor can be easily fabricated, at a low cost and in large quantities. Consequently, the sensor of the present invention has superior industrial applicability.
In this method, it is preferable that the fracture piece is directly joined to the substrate at the opposite end portions or indirectly joined to the substrate at the opposite end portions via a bonding layer by electric resistance welding.
With such a procedure, since the fracture piece is directly joined to the first surface of the substrate at the opposite end portions or indirectly joined to the first surface at the opposite end portions via the bonding layer, the strain can be reliably transmitted from the substrate to the opposite end portions of the fracture piece and the tensile stress can be concentratively generated in the central portion of the fracture piece without being dispersed, which facilitates occurrence of the crack.
Also, in this method, it is preferable that a ratio L3/L4 of a length L3 of the intermediate portion of the fracture piece in the longitudinal direction to a length L4 of an unjoined region between the opposite end portions in the longitudinal direction is used to adjust sensitivity in such a manner that the sensitivity is made higher as the ratio L3/L4 is decreased.
With such a procedure, by suitably adjusting the length L3 of the intermediate portion with respect to the length L4 of the unjoined region to increase or decrease the ratio L3/L4, the sensitivity can be arbitrarily set. For example, when the ratio L3/L4 is decreased, the sensitivity is made higher. Therefore, the fatigue detecting sensor having high precision and desired sensitivity is realized. The fatigue detecting sensor having the desired sensitivity can be easily fabricated because the length L of the intermediate portion can be adjusted without special treatment.
There is further provided a method for estimating damage using a crack-type fatigue detecting sensor comprising the steps of: fixing the above-described crack-type fatigue detecting sensor to a member to be tested; measuring a length of crack propagation during a predetermined period; and estimating damage of the member to be tested based on the length of crack propagation.
With such a procedure, the length of the crack propagation can be measured by using the crack-type fatigue detecting sensor with high sensitivity and high precision, and based on the measured length, fatigue damage of the member to be tested is estimated. Therefore, even fatigue caused by the small strain in an initial stage can be detected, and based on the length of the crack with high sensitivity, high precision, and high reliability, the fatigue damage of the member can be estimated.